The Met Office has issued a warning for 36 hours of heavy rain across much of Wales from Tuesday.
The yellow runs from 6am on Tuesday until 6pm on Wednesday, with up to 200mm of rainfall possible in some locations.
Forecasters say flooding of a'few homes and busineses is likely' and the heavvy rainfall will affect journeys and is likely to cause interruption to power supplies.
The Met Office says: "Outbreaks of rain, heavy at times, will affect many parts of the UK on Tuesday and Wednesday.
"The heaviest and most persistent rainfall during this period is expected to affect the high ground of northwest England, northwest Wales and south Wales.
"Over the course of this 36 hours in these areas, 50-100 mm of rainfall is likely to accumulate widely, with as much as 150-200 mm in some locations."